The present invention relates to an optical semiconductor device having a strain compensation quantum well structure including a compression strain quantum well layer and an extension strain barrier layer, and a method for fabricating the same.
Recently, a surface emitting type semiconductor laser device expected to have a large number of advantages, over an edge emitting type semiconductor laser device, such as a low operation current, a high degree of integration through two-dimensional array arrangement and a low cost is being earnestly developed. In particular, use of a gallium arsenic (GaAs)-based compound semiconductor material with an oscillation wavelength of 850 nm or more has been studied and developed for short-distance fast optical communication such as Gigabit Ethernet (registered trademark) or optical data links, and such technique has already been commercialized.
A previously reported surface emitting type semiconductor laser device of a 850 nm band employs, as an active layer, an aluminum gallium arsenide (AlGaAs)-based multiple quantum well structure (for example, see IEEE Photon. Technol. Lett., vol. 3, 1991, pp. 859-862). In this conventional device, for example, GaAs is used as a well layer and Al0.3Ga0.7As is used as a barrier layer.
As means for further increasing optical gain in an active layer of a semiconductor laser device and for realizing a smaller threshold current in the resultant semiconductor laser device, what is called a strain compensation quantum well active layer including a well layer provided with compression strain and a barrier layer provided with extension strain has recently been proposed (for example, see Japanese Laid-Open Patent Publication No. 9-162482).
When a well layer of the quantum well structure is provided with the compression strain, degeneration of a valence band is relieved owing to the compression effect, and hence, the state density of holes is lowered. The lowering of the state density of holes reduces the effective mass of the holes and the holes can be easily recombined with electrons. Therefore, the optical gain is increased, resulting in lowering the threshold current and increasing the efficiency of optical output. In addition, when the barrier layer is provided with the extension strain, the compression strain of the well layer is relieved, so that crystallinity degradation such as misfit dislocation can be suppressed. However, even when the Al composition of an AlGaAs-based material is changed, its lattice constant is minimally changed, and hence stress strain is minimally caused in the active layer.
Therefore, for realizing the oscillation frequency of 850 nm, a strain compensation quantum well active layer using, as the well layer, AlxGayIn1-x-yAs (wherein 0<x, y<1 and 0<x+y<1) obtained by adding indium (In) to an AlGaAs-based compound semiconductor and, as the barrier layer, AlxGa1-xAsyP1-y (wherein 0<x and y<1) obtained by adding phosphorus (P) to the AlGaAs-based compound semiconductor has been proposed. When such a quantum well active layer with stress strain is used, a surface emitting type semiconductor laser device with a small threshold current can be realized.